Publications

4508

‘Candidatus Liberibacter asiaticus’ (Las) is an unculturable, phloem-limited, insect-transmitted bacterium associated with the Asiatic form of huanglongbing (HLB), the most destructive citrus disease. In Asia and the Americas, it is transmitted by the Asian citrus psyllid (Diaphorina citri Kuwavama). Despite considerable research, little is known about the processes involved in plant infection and colonization by Las. This study was conducted to determine whether the basal portion (below girdling) of the plant is an important route for Las to move laterally from a point of inoculation on a branch to pathogen-free branches elsewhere in the canopy, and to quantify the influence of actively growing tissues on vertical upward (acropetally) or downward (basipetally) movement of Las. Nongirdled and fully or partially girdled stems of potted plants of ‘Pera’ sweet orange, graft-inoculated above or below girdling, were sampled in distinct regions and assessed by qPCR, 6 months postinoculation. Las invaded all regions of partially and nongirdled plants but remained restricted to the inoculated regions of fully girdled plants, evidence that in planta bacterium movement is limited to the phloem. In fully girdled plants, starch accumulated above the girdling site, probably because of changes in flow of phloem sap. To study the influence of actively growing tissues, inoculated ‘Valencia’ sweet orange plants were kept intact or were top- or root-pruned to force production of new tissues, and sampled at 15-day intervals. Las migrated rapidly and most predominantly toward newly developing root and leaf tissues. The rapid and predominant movement of Las to newly developed shoots and roots would explain failures of canopy heat treatments and pruning to cure HLB-affected trees, and reinforces the need to protect rapidly growing new shoots from feeding by D. citri in order to minimize transmission and spread of the pathogen by the vector within and between orchards.

Sugarcane Grassy Shoot (SCGS) disease is known to be related to Rice Yellow Dwarf (RYD) phytoplasmas (16SrXI-B group) which are found predominantly in sugarcane growing areas of the Indian subcontinent and South-East Asia. The 16S rRNA gene sequences of SCGS phytoplasma strains belonging to the 16SrXI-B group share 98.07 % similarity with ‘Ca. Phytoplasma cynodontis’ strain BGWL-C1 followed by 97.65 % similarity with ‘Ca. P. oryzae’ strain RYD-J. Being placed distinctly away from both the phylogenetically related species, the taxonomic identity of SCGS phytoplasma is unclear and confusing. We attempted to resolve the phylogenetic positions of SCGS phytoplasma based on the phylogenetic analysis of 16S rRNA gene (>1500 bp), nine housekeeping genes (>3500 aa), core genome phylogeny (>10 000 aa) and OGRI values. The draft genome sequences of SCGS phytoplasma (strain SCGS) and Bermuda Grass White leaf (BGWL) phytoplasma (strain LW01), closely related to ‘Ca. P. cynodontis’, were obtained. The SCGS genome was comprised of 29 scaffolds corresponding to 505 173 bp while LW01 assembly contained 21 scaffolds corresponding to 483 935 bp with the fold coverages over 330× and completeness over 90 % for both the genomes. The G+C content of SCGS was 19.86 % while that of LW01 was 20.46 %. The orthoANI values for the strain SCGS against strains LW01 was 79.42 %, and dDDH values were 22. Overall analysis reveals that SCGS phytoplasma forms a distant clade in RYD group of phytoplasmas. Based on phylogenetic analyses and OGRI values obtained from the genome sequences, a novel taxon ‘Candidatus Phytoplasma sacchari’ is proposed.

Abstract Phy.to.plas‐ma. Gr. masc. n. phytos a plant; N.L. neut. n. Phytoplasma a form from a plant. Phytoplasmas are wall‐less, nutritionally fastidious, and phytopathogenic prokaryotes 0.2–0.8 μm in diameter that morphologically resemble nonhelical members of the class Mollicutes . Sequencing of nearly full‐length PCR‐amplified 16S rRNA genes, combined with earlier studies, provided the first comprehensive phylogeny of the organisms and showed that they constitute a unique, monophyletic clade within the Mollicutes . These organisms are most closely related to members of the genus Acholeplasma within the Anaeroplasma clade as defined by Weisburg et al. Despite decades of efforts and recent progress, sustained pure culture in cell‐free media has not yet been demonstrated for any phytoplasma. Their genome sizes have been estimated to range from 530 to 1,350 kb, and the DNA G + C content is about 23–30 mol%. The presence of a characteristic oligonucleotide sequence in the 16S rRNA gene, CAA GAY BAT KAT GTK TAG CYG GDC T, and standard codon usage indicate that phytoplasmas represent a distinct genus‐level taxon for which the name ‘ Candidatus Phytoplasma’ has been adopted. At present, for any subtaxa within the provisional genus, the designation ‘ Candidatus ’ must still be used. DNA G + C content (mol%) : 23–29. Taxonomic and Nomenclature Notes According to the List of Prokaryotic names with Standing in Nomenclature (LPSN), the taxonomic status of the genus Candidatus Phytoplasma is: preferred name (not correct name) (last update, February 2025) * . LPSN classification: Bacteria / Bacillati / Mycoplasmatota / Mollicutes / Acholeplasmatales / Acholeplasmataceae / Candidatus Phytoplasma Candidatus Phytoplasma could not be recovered in GTDB ** . * Meier‐Kolthoff et al. ( 2022 ). Nucleic Acids Res , 50 , D801 – D807 ; DOI: 10.1093/nar/gkab902 ** Parks et al. ( 2022 ). Nucleic Acids Res , 50 , D785 – D794 ; DOI: 10.1093/nar/gkab776

Abstract Ther.mo.ha.lo.bac.te.ra.ce'ae. N.L. masc. n. Thermohalobacter , type genus of the family; suff. ‐aceae , ending to denote a family; N.L. fem. pl. n. Thermohalobacteraceae , the Thermohalobacter family. Free‐living bacteria that mainly occur in anoxic saline environments. Cells are not pigmented and have a Gram‐positive type of cell wall. Gram‐stain reaction can be negative or positive. Typically slender rod‐shaped cells that are motile by flagella. Several strains form heat‐resistant spores. Mainly euryhaline, moderately thermophilic and halophilic. Members of the family are strictly anaerobic bacteria that utilize carbohydrates or peptides by fermentation. Cellular fatty acid patterns are dominated by iso ‐C 15:0 and iso ‐C 15:0 DMA. The family currently accommodates members of the genera Thermohalobacter , Caldisalinibacter , Caloranaerobacter , Brassicibacter , Sporosalibacterium , and Clostridiisalibacter , which were previously affiliated with the family Clostridiaceae . The affiliation of novel species to this family depends on the phylogenetic position, which should be determined on the basis of comparative sequence analyses of 16S rRNA genes or genomes. DNA G + C content (mol%): 28–33. Type genus: Thermohalobacter Cayol et al. 2000. Taxonomic and Nomenclature Notes According to the List of Prokaryotic names with Standing in Nomenclature (LPSN), the taxonomic status of the family Thermohalobacteraceae is: synonym (last update, February 2025) * . Correct name: Clostridiaceae LPSN classification: Bacteria / Bacillati / Bacillota / Clostridia / Eubacteriales / Clostridiaceae The family Thermohalobacteraceae can also be recovered in the Genome Taxonomy Database (GTDB) as f__Thermohalobacteraceae (version v220) ** . GTDB classification: d__Bacteria / p__Bacillota_A / c__Clostridia / o__Tissierellales / f__Thermohalobacteraceae * Meier‐Kolthoff et al. ( 2022 ). Nucleic Acids Res , 50 , D801 – D807 ; DOI: 10.1093/nar/gkab902 ** Parks et al. ( 2022 ). Nucleic Acids Res , 50 , D785 – D794 ; DOI: 10.1093/nar/gkab776

Tick-borne agents constitute a growing concern for human and animal health worldwide. Hyalomma aegyptium is a hard tick with a three-host life cycle, whose main hosts for adults are Palearctic tortoises of genus Testudo. Nevertheless, immature ticks can feed on a variety of hosts, representing an important eco-epidemiological issue regarding H. aegyptium pathogens circulation. Hyalomma aegyptium ticks are vectors and/or reservoirs of various pathogenic agents, such as Ehrlichia, Anaplasma, Babesia and Hepatozoon/Hemolivia. Ehrlichia and Anaplasma are emergent tick-borne bacteria with a worldwide distribution and zoonotic potential, responsible for diseases that cause clinical manifestations that grade from acute febrile illness to a fulminant disease characterized by multi-organ system failure, depending on the species. Babesia and Hepatozoon/Hemolivia are tick-borne parasites with increasing importance in multiple species. Testudo graeca tortoises acquired in a large animal market in Doha, Qatar, were screened for a panel of tick-borne pathogens by conventional PCR followed by bidirectional sequencing. The most prevalent agent identified in ticks was Hemolivia mauritanica (28.6%), followed by Candidatus Midichloria mitochondrii (9.5%) and Ehrlichia spp. (4.7%). All samples were negative for Babesia spp. and Hepatozoon spp. Overall, 43% of the examined adult ticks were infected with at least one agent. Only 4.7% of the ticks appeared to be simultaneously infected with two agents, i.e., Ehrlichia spp. and H. mauritanica. This is the first detection of H. mauritanica, Ehrlichia spp. and Candidatus M. mitochondrii in H. aegyptium ticks collected from pet spur-thighed tortoises, in Qatar, a fact which adds to the geographical extension of these agents. The international trade of Testudo tortoises carrying ticks infected with pathogens of veterinary and medical importance deserves strict control, in order to reduce potential exotic diseases.